Right after I finished mulching my rose garden with free wood chips I got from a tree service, I read that wood chips suck all the nitrogen out of the soil. Should I fertilize now?
As soon as you spread mulch, there’s a population boom of bacteria, fungi, and other soil microbes that work to decompose organic matter. These beneficial organisms consume some of the nitrogen present in the soil. The nitrogen loss is temporary, however, because the nitrogen is once again available to plants when the microbes die.
But in the meantime, the thin layer of soil that is in direct contact with the mulch will relinquish some of its fertility. The depletion of nutrients does not extend deep into the soil, so it won’t likely affect anything other than the shallowest-rooted annuals.
The benefits of organic mulch outweigh the possibility of a temporary nitrogen shortage. Mulch conserves moisture and contributes to a healthy root zone. It also helps to suppress weeds. As mulch decomposes, the nutrients held within it—including a small amount of nitrogen—are released.
For roses and other plants with high nutrient needs, it’s a good idea to keep an eye out for nitrogen deficiency, whether you’ve mulched with wood chips or not. Plants lacking nitrogen grow slowly and exhibit pale or yellow-green leaves; in extreme cases, growth may be stunted.
To preclude the chance of nitrogen deficiency, many gardeners make a habit of distributing a small dose of slow-release nitrogen fertilizer every spring to roses, perennials, edible crops, and other heavy feeders. Good organic sources include alfalfa meal (about 5 percent nitrogen by weight), fish meal or fish emulsion (5 percent), blood meal (12 percent), and feather meal (15 percent). Compost and worm castings also contain small amounts of nitrogen. —Doug Hall
Please define organic where using superphosphate is concerned. Also when using 10-10-10. Thank you.
Superphosphate is a synthetic chemical fertilizer that is not approved for use in organic agriculture. It is manufactured by treating mined phosphate rock with sulfuric acid. It’s toxic to the microbial life of your soil. Because it’s water soluble, it can also contribute to phosphorus pollution of waterways.
If your garden soil lacks phosphorus, it’s better to apply it in the form of bone meal, rock phosphate, composted poultry manure, or compost made from yard waste and kitchen scraps. These organic materials release phosphorus to the soil slowly as they are broken down by soil bacteria. Fish emulsion and liquid seaweed fertilizers offer a quick phosphorus boost.
The ratio 10-10-10 describes the quantities of nutrients within a fertilizer but doesn’t reveal whether or not it’s organic. That said, the vast majority of fertilizers with a 10-10-10 analysis are synthetic. The three numbers represent the percentages of available nitrogen, phosphorus, and potassium in the fertilizer.
Gardeners who are in the habit of regularly feeding their soil with rich compost often find that they have no need for additional fertilizers. If necessary, you can supplement compost with complete organic fertilizers that blend natural sources of nitrogen, phosphorus, and potassium, as well as micronutrients such as iron and calcium. You’ll know which products at the garden center are organic by their “OMRI Listed” label. —Doug Hall